The characterization of vitreous materials and articles made therefrom, e.g. glass fibers, for example optical fibers for use in communication, requires the knowledge of various parameters of the material including the softening point.
The softening point of a vitreous material can be defined as the temperature at which the material attains a viscosity level at which it can no longer be considered a solid.
Generally speaking vitreous material cannot be considered to have a change of state from the solid to a liquid at a single well-defined temperature. Consequently, one refers to the temperature range at which the viscosity drops from a high viscosity to a low viscosity as a softening point.
In the viscosity-temperature plane, there is a first viscosity zone, a second low-viscosity zone and a third zone comprising the softening point located between them.
In the fabrication and handling of optical fibers, the softening point must be determined in order to evaluate the possibility of carrying out operations such as the fabrication of the so-called fusion splices, i.e. connections between two optical fibers effected by heating the ends to be joined to the softening point and then bringing them together.
Where the fibers are composed of different materials, a disparity in their softening points may pose a significant problem. In such cases fiber compatibility is poor and residual stresses remain in the fusion zone which is very fragile.
One apparatus for determining the softening point of a glass fiber is described in TutorialSymposium Proceedings, New York, 8-9 June 1970, pages 355-357.
A cylinder or rod of a given length of the glass to be measured is suspended in a furnace and its increase in length as function of temperature is determined by suitable detectors.
The temperature is increased in accordance with a particular law and the softening point is determined as the temperature at which the lengthening rate of the fiber attains a predetermined value. This apparatus is characterized by low reproducibility. When the test specimen reaches the softening point, it begins to lengthen, but also its cross section decreases and surface strains and the rate of increase in temperature play a role. Density and surface tension of the test glass also play a role. The results are, therefore, not always reliable and are seldom reproducible, especially for higher temperatures and greater decrease of the lengthening rate.